The present invention relates to the art of nanotechnology, and in particular, to carbon nanotube technology, its function and structure.
A carbon nanotube is a single graphene sheet in the form of a seamless cylinder. The ends of a nanotube typically have hemispherical caps. The typical diameter of a nanotube ranges from about 1 nm to 10 nm. The length of a nanotube potentially can be millions of times greater than its diameter.
Since their discovery in the early 1990s, carbon nanotubes have been the focus of intense study due to their very desirable and unique combination of physical properties. They are chemically inert, thermally stable, highly strong, lightweight, flexible and electrically conductive. In fact, carbon nanotubes may potentially be stiffer and stronger than any other known material.
Carbon nanotubes are currently being proposed for numerous applications, such as, for example, catalyst supports in heterogeneous catalysis, high strength engineering fibers, sensory devices and molecular wires for the next generation of electronics devices.
There has been particularly intense study of the electrical properties of nanotubes, and their potential applications in electronics. Metallic carbon nanotubes have conductivities and current densities that meet or exceed the best metals; and semiconducting carbon nanotubes have mobilities and transconductance that meet or exceed the best semiconductors.
Carbon nanotubes are grown by combining a source of carbon with a catalytic nanostructured material such as iron or cobalt at elevated temperatures. At such temperatures, the catalyst has a high solubility for carbon. The carbon links up to form graphene and wraps around the catalyst to form a cylinder. Subsequent growth occurs from the further addition of carbon.
A problem with the synthesis of carbon nanotubes is that the as-prepared material frequently contains a number of metallic and amorphous impurities, interfering with their reliable and optimal usage in applications such as field emission displays, molecular computers, and ultrahigh strength materials. Current techniques for purifying carbon nanotubes are based on bromination, plasma etching, chromatographic separation, cascade membrane microfiltration, sonication and oxidation of nanotubes.
The current processes for oxidizing nanotubes are acid reflux in solution or gaseous oxidation. Although these oxidative processes are capable of generating a variety of oxygenated functional groups, such as aldehydic, ketonic, esteric, alcoholic, and carboxylic moieties, these processes do not allow for rationally controlling the type of oxygen moieties placed on the nanotubes. Without control over the types of oxygen moieties, subsequent chemical derivatization and modulation of electronic and mechanical properties of the nanotubes is difficult.
Moreover, current oxidative processes only allow for oxygen moieties to be placed at the ends of the nanotubes, and at structural defect sites along the nanotube walls. With such restricted placement of oxygen moieties on nanotubes, the end-use applications of the carbon nanotubes are limited. Also, the high level of purity needed for various end-use applications of nanotubes is substantially undermined.
Furthermore, current oxidative processes are performed with nanotubes in the gaseous phase. High temperatures are required for such processes which may lead to damaging consequences, such as destroying the structural integrity of nanotubes and low product yield. Moreover, current oxidative processes tend to etch away from the defect sites and ends of nanotubes, thereby cutting nanotubes into short fragments. By etching nanotubes in such a manner, the aspect ratio of nanotubes cannot be maintained, let alone maximized.
Thus, the current methods for purifying carbon nanotubes involve unreliable, low-yield processes. Also, current methods for adding oxygen moieties to carbon nanotubes are unpredictable with respect to the location of the moieties, and the types of moieties. These shortcomings of current methods present obstacles for actualizing the utility of carbon nanotubes for end-use applications.
Accordingly, there remains a need for a reliable method of providing carbon nanotubes which have a high level of purity. Also, there remains a need for rationally controlling the location and types of oxygen moieties placed on carbon nanotubes.